Alzheimer’s disease

In population studies, high dietary intake of fat and calories was associated with an increased risk for Alzheimer’s disease, whereas high intake of fish was associated with a decreased risk. Whether these associations represent a cause-and-effect relationship is unknown.

Angina

Fish oil, which contains the beneficial fatty acids known as EPA and DHA, has been studied in the treatment of angina. In some studies, 3 grams or more of fish oil three times per day (providing a total of about 3 grams of EPA and 2 grams of DHA) have reduced chest pain as well as the need for nitroglycerin, a common medication used to treat angina; other investigators could not confirm these findings. It is not known if eating fish will confer any benefits for people with angina.

Asthma

There is evidence that children who eat oily fish may have a much lower risk of getting asthma. Moreover, in a double-blind trial, children who received 300 mg per day of fish oil (providing 84 mg of EPA and 36 mg of DHA) experienced significant improvement of asthma symptoms. It should be noted that these benefits were obtained under circumstances in which exposure to food allergens and environmental allergens was strictly controlled. Though the evidence supporting the use of fish oils remains somewhat conflicting, eating more fish and supplementing with fish oil may still be worth considering, especially among children with asthma.

Atherosclerosis

Supplementation with fish oil, rich in omega-3 fatty acids, has been associated with favorable changes in various risk factors for atherosclerosis and heart disease in some, but not all, studies. A double-blind trial showed that people with atherosclerosis who took fish oil (6 grams per day for 3 months and then 3 grams a day for 21 months) had significant regression of atherosclerotic plaques and a decrease in cardiovascular events (e.g., heart attack and stroke) compared to those who did not take fish oil. These results contradict the findings of an earlier controlled trial in which fish oil supplementation for two years (6 grams per day) did not promote major favorable changes in the diameter of atherosclerotic coronary arteries. It is not known if eating fish will confer similar benefits. However, the original fish oil research was stimulated by population studies that showed a remarkably low incidence of atherosclerosis among Inuit people, whose diets contain large quantities of fresh fish.

Bipolar disorder

People diagnosed with depression may have lower blood levels of omega-3 fatty acids. A recent double-blind trial evaluated the effect of omega-3 fatty acid supplementation on people with bipolar disorder. Patients in this study were given 9.6 grams of omega-3 fatty acids from fish oil, or a placebo, in addition to their conventional medications. Those receiving this large amount of fish oil had significant improvements compared to those taking placebo, including a longer period between relapses. It is not known if eating fish will confer similar benefits.

Bronchitis

Dietary factors may influence both inflammatory activity and antioxidant status in the body. Increased inflammation and decreased antioxidant activity may each lead to an increased incidence of chronic diseases, such as chronic bronchitis. People suffering from chronic bronchitis may experience an improvement in symptoms when consuming a diet high in anti-inflammatory fatty acids, such as those found in fish.

Cancer

Fish eaters have been reported to have low risks of cancers of the mouth, throat, stomach, colon, rectum, pancreas, lung, breast, and prostate. The omega-3 fatty acids found in fish are thought by some researchers to be the components of fish responsible for protection against cancer. Many doctors recommend fish as part of an anticancer diet.

Fish Oil Supplements and the Treatment of People with Weight Loss Associated with Late-Stage Cancer

In a controlled trial, late-stage cancer patients were given fish oil supplements, which increased patients’ survival time by several months. Among those patients who were terminal or near terminal at the beginning of the trial, immune status also improved. It is not known if eating fish would produce similar effects.

In a preliminary report, 18 late-stage pancreatic cancer patients were given fish oil supplements. Before supplementation, weight loss averaged over six pounds per month. After three months of supplementation, patients had gained an average of two-thirds of a pound per month. Fish oil supplementation appears to be a promising therapy for slowing weight loss in late-stage cancer patients. It is not known if eating fish would produce similar effects.

Cancer (Breast)

Fish is a source of a type of dietary fat considered potentially helpful in protecting against breast cancers.

Why do some research findings suggest fat increases the risk of cancer and other studies find no association? Some studies finding dietary fat unrelated to cancer risks have not factored out the effects of olive oil and fish fat. Both may protect against cancer. Adding them to the total dietary fat intake and then studying whether “more fat causes more cancer” is, therefore, a mistake. Some studies finding no association between fat intake and breast cancer have made this error.

Scientists know cancers caused by diet most likely occur many years after the causative food is consumed. When researchers compared dietary intakes to cancer rates occurring ten years after the consumption of food, and also factored out the effect of fat from fish consumption, they found a high degree of correlation between consumption of animal fat (other than from fish) and the risk of breast cancer death rates for women at least 50 years of age.

In the debate over whether dietary fat increases breast cancer risks, only one fact is indisputable: women in countries that consume high amounts of meat and dairy fat have a high risk of breast cancer, while women in countries that mostly consume fish (instead of dairy fat and meat) have a low risk of breast cancer.

Cancer (Colon)

Several human studies have found that supplementation with fish oil leads to a reduction in markers for the risk of colon cancer. Despite these promising reports, no trial has yet investigated whether dietary fish consumption or continuous supplementation with fish oil would actually help prevent colon cancer, or effectively treat people who already have been diagnosed with colon cancer.

Cardiac arrhythmia

In a double-blind trial, people with a type of arrhythmia known as ventricular premature complexes were given fish oil supplements or a similar placebo. Persons taking the fish oil had a significantly reduced frequency of abnormal heartbeats, compared to those receiving placebo. It is not known if eating fish would produce similar results.

Chronic Obstructive Pulmonary Disease (COPD)

A greater intake of the omega-3 fatty acids found in fish and fish oils has been linked to reduced risk of COPD, though research has yet to investigate whether dietary fish consumption or fish oil supplements would help people with COPD.

Crohn’s disease

Inflammation within the gut occurs in people suffering from Crohn’s disease. EPA and DHA, the omega-3 fatty acids found in fish and fish oil, have anti-inflammatory activity. A two-year trial compared the effects of having people with Crohn’s disease eat 3.5 to 7 ounces of fish high in EPA and DHA per day or a diet low in fish. In that trial, the fish-eating group had a 20% relapse rate compared with a 58% rate in those not eating fish. Salmon, herring, mackerel, albacore tuna, and sardines are all high in EPA and DHA. Some types of fish oil supplements may also be helpful.

Depression

The amount and type of dietary fat consumed may influence the incidence of depression. A high intake of omega-6 fatty acids relative to omega-3 fatty acids and an inadequate intake of omega-3 fatty acids (e.g., from fish and fish oils) have both been associated with increased levels of depression. People who eat diets high in omega-3 fatty acids from fish have a lower incidence of depression and suicide.

Omega-3 fatty acids found in fish and fish oil, particularly DHA, are needed for normal nervous system function. Depressed people have been reported to have lower omega-3 fatty acid levels (e.g., DHA) than people who are not depressed. Low levels of the other omega-3 fatty acid from fish, EPA, have correlated with increased severity of depression. In a double-blind trial, people with manic depression were given a very high intake of supplemental fish oil for four months. Although some scores of depression levels fell as much as 48% in the omega-3 fatty acid group, it is not known if eating fish could produce similar results, because it is a far less concentrated source of EPA and DHA.

Diabetes

Eating fish may afford some protection from diabetes. Incorporating a fish meal into a weight-loss regimen was effective at improving glucose and insulin metabolism and lowering high cholesterol.

Glucose tolerance improves in healthy people taking fish oil supplements. However, other studies report that cholesterol increases and diabetes worsens with fish oil supplements.

Until this issue is resolved, people with diabetes should feel free to increase their fish intake, but they should consult a doctor before taking omega-3 fish oil supplements.

Dysmenorrhea

Diets low in omega-3 fatty acids (EPA and DHA) from fish and fish oil have been associated with more menstrual pain. Fish oil supplements have helped relieve dysmenorrhea in some studies. However, it is not known if eating fish would produce a similar effect.

Eczema

Fish oil supplements have been shown to be helpful for people with eczema. It is not known whether eating fish rich in omega-3 fatty acids (EPA and DHA) would have similar effects.

Endometriosis

Animal research suggests that fish oils may reduce the severity of endometriosis, and fish oils have been shown to improve symptoms of dysmenorrhea (painful menstruation), which may be caused by endometriosis. However, it is not known whether eating fish would have similar effects.

Fibrocystic breast disease

Fibrocystic disease has been linked to excess estrogen. When those with fibrocystic disease are put on a low-fat diet, their estrogen levels decrease. After three to six months, the pain and lumpiness also decrease. The link between fat and symptoms appears to be most strongly related to saturated fat. Foods high in saturated fat include meat and dairy products. Fish is a possible replacement.

Gestational hypertension (GH)

Increased consumption of fish has been associated with reduced risk of GH in one preliminary trial. In this study, the incidence of GH was significantly higher in women from communities with lower consumption of fish, and lower in the communities with high fish consumption. Current evidence is insufficient to recommend the use of fish oil in GH.

Halitosis

Access by oral bacteria to sulfur-containing amino acids will enhance the production of sulfur gases that are responsible for bad breath. This effect was demonstrated in a study in which concentrations of these sulfur gases in the mouth were increased after subjects used a mouth rinse containing the amino acid cysteine. Cleaning the mouth after eating sulfur-rich foods such as fish may help remove the food supporting these bacteria.

Heart attack

Several trials report that eating fish decreases heart attack deaths and reduces the size of the heart injury, though some researchers have not confirmed these findings. The link between fish eating and heart attack prevention is supported by research showing that fish oil supplements help reverse atherosclerosis.

Making positive dietary changes immediately following a heart attack is likely to decrease one’s chance of having a second heart attack. In one study, people began eating more vegetables and fruits, and substituted fish, nuts, and legumes for meat and eggs 24–48 hours after a heart attack. Six weeks later, the diet group had significantly fewer fatal and nonfatal heart attacks than a similar group who did not make these dietary changes. This trend continued for an additional six weeks.

Many doctors tell people trying to reduce their risk of heart disease to avoid all meat, margarine, and other processed foods containing hydrogenated oils and dairy fat. Fish are often suggested instead of meat.

Fish oil supplementation has led to partial reversal of atherosclerosis and a reduction in incidence of heart attacks in double-blind trials. Fish oil may reduce the amount of heart muscle damage from a heart attack and enhance the effect of blood-thinning medication.It is not known if eating fish would produce similar results.

High cholesterol

Eating fish has been reported to increase HDL cholesterol and is linked to a reduced risk of heart disease in most but not all studies. Fish contains very little saturated fat, and fish oil contains EPA and DHA, omega-3 fatty acids that appear to protect against heart disease.

High triglycerides

Some, but not all, studies have found that increasing consumption of fish is associated with a lower risk of heart disease. Significant amounts of TG-lowering omega-3 fatty acids (EPA and DHA) can be found in the flesh and oil of salmon, herring, mackerel, sardines, anchovies, albacore tuna, and black cod. Many doctors recommend that people with elevated TGs increase their intake of these fatty fish.

Many double-blind trials have consistently demonstrated that fish oil supplements lower TG levels. Cod-liver oil, another source of omega-3 fatty acids, has also been found to lower TGs. It is not known if eating fish can effectively reduce TG levels.

Hypertension

EPA and DHA, the omega-3 fatty acids found in fish oil, lower blood pressure, according to an analysis of 31 trials. The effect was dependent on the amount of omega-3 oil used, with the best results occurring in trials using unsustainably high levels such as 15 grams per day—the amount often found in 50 grams of fish oil. It is not known if eating any amount of fish can control hypertension. In any case, it would be impossible to eat enough fish on a daily basis to provide an equivalent amount of omega-3 fatty acid as was used in the studies cited above.

Immune function

Research on the immune effects of the omega-3 fatty acids from fish and fish oils is conflicting. Liquid diets containing omega-3 fatty acids used in hospitals for critically ill people have been shown to improve immune function and reduce infections. However, in one controlled study in healthy people, a low-fat diet improved or maintained immune function, but when fish was added to increase omega-3 fatty acid intake, immune function was significantly inhibited.

Insulin resistance syndrome (IRS)

The effect of dietary fat on insulin resistance seems to depend on the type of fat eaten. Preliminary studies in animals and humans suggest that insulin resistance is worsened with increased use of saturated fat and improved with increased unsaturated omega-3 fatty acids from fish. Recently, a low-fat diet allowing fish was shown to decrease insulin resistance in people with IRS.

In two controlled studies, a combined program of a weight-loss diet lower in fat and higher in fish, along with exercise three times per week, improved several measures of insulin resistance, blood triglycerides and cholesterol, and blood pressure in a group of people with IRS.

Iron-deficiency anemia

Iron deficiency is not usually caused by a lack of dietary iron alone. Nonetheless, a lack of iron in the diet is often part of the problem, so ensuring an adequate supply of iron is important. The most absorbable form of iron, called “heme” iron, is found in meat, poultry, and fish. Non-heme iron is also found in these foods.

Lupus (SLE)

Foods high in omega-3 fatty acids, such as fish, may decrease lupus-induced inflammation.

The omega-3 fatty acids in fish oil decrease inflammation and may be beneficial for persons with SLE. To date, however, all studies on fish oil have used supplements and not fish. Nonetheless, many doctors recommend their SLE patients eat several servings of fatty fish each week.

Macular degeneration

According to preliminary research, people who eat fish more than once per week have half the risk of developing age-related macular degeneration compared to people who eat fish less than once per month.

Migraine headaches

Fish oil containing EPA and DHA may reduce the symptoms of migraine headaches. It is not known if eating fish would have similar effects.

Multiple sclerosis (MS)

A survey of people in 36 different countries found that people who ate more fish had less risk of developing MS, while those who ate pork, hot dogs, and other foods high in animal (saturated) fats were at greater risk. Although some doctors recommend fish oil capsules for people with MS, few investigations have explored the effects of this supplement.

Night blindness

Night blindness can be an early sign of vitamin A deficiency. Such a deficiency can result from inadequate intake of animal foods (the main source of vitamin A), including some fish.

Osteoporosis

A preliminary trial found that elderly women with osteoporosis who were given supplements containing 4 grams of fish oil per day for four months had improved calcium absorption and resulted in evidence of new bone formation. It is not known if eating fish would have similar effects.

Pancreatic insufficiency

A few preliminary reports suggest that food allergy may cause some cases of acute pancreatitis. Food allergies identified in these cases included fish and fish eggs. No research has investigated the possible role of food allergy in other causes of pancreatic insufficiency.

Parasites

Undercooked fish can contain parasites.

Phenylketonuria (PKU)

A PKU diet is low in protein, providing no more than the minimum amount of phenylalanine needed by the body. All high-protein foods, including fish, are usually eliminated.

Photosensitivity

In a preliminary study, supplementation with fish oil for three months reduced photosensitivity in nine of ten people suffering from polymorphous light eruptions. It is not known if eating fish would help with this condition.

Preeclampsia

Fish oil supplementation has been proposed to lower the incidence of preeclampsia. However, blinded trials suggest that fish oil does not reduce symptoms or protect against preeclampsia. The effects of dietary fish consumption on preeclampsia are not known. Supplementation with fish oil significantly reduced recurrence of premature delivery, according to data culled from six clinical trials involving women with a high risk for such complications. Fish oil supplementation did not prevent premature delivery of twin pregnancies, nor did it have any preventive effect against intrauterine growth retardation or pregnancy-induced hypertension. Fish oils should be free of contaminants, such as mercury and organochlorine pesticides. It is not known if eating fish would have similar health benefits.

Women who eat substantial amounts of certain types of seafood (e.g., swordfish, tuna) may be ingesting contaminants that can increase the risk of brain and nervous system abnormalities in their offspring. Exposure to mercury and polychlorinated biphenyls (PCBs) was found to be increased in relation to maternal intake of seafood. Higher exposure to these toxic contaminants has been linked to an increased risk of deficits in the developing brains and nervous systems of the children.

Psoriasis

In double-blind trials, fish oil and purified eicosapentaenoic acid (EPA, one of the fatty acids found in fish oil) supplements have improved psoriasis. It is not known if eating fish would have similar effects.

Raynaud’s disease

In a double-blind study, supplementation with fish oil for 6 or 12 weeks reduced the severity of blood-vessel spasm in people with Raynaud’s phenomenon. Fish oil was effective in people with primary Raynaud’s disease, but not in those whose symptoms were secondary to another disorder. It is not known if eating fish would have similar effects.

Rheumatoid arthritis (RA)

Many double-blind trials have proven that omega-3 fatty acids in fish and fish oil, called EPA and DHA, partially relieve symptoms of RA. The effect results from the anti-inflammatory activity of fish oil. It is not known if eating fish would have similar effects.

Rickets

Fish liver oil is among the few foods that contain vitamin D. Vitamin D is required to prevent rickets, a disease of vitamin D deficiency.

Ulcerative colitis (UC)

Because ulcerative colitis is an inflammatory condition and fish oil has anti-inflammatory activity, fish oil has been tested as a potential remedy for people with UC. Clinical trials of fish oil for UC have yielded mixed results. It is not known if eating fish would provide any benefits for persons with UC.